With the advent of digital computing, including digital photography, there has been an explosion in the amount of digital images, such as photographs and video, that are generated and transmitted by electronic imaging equipment, such as digital cameras, camcorders, copiers, scanners, PDAs (Personal Digital Assistant), cell-phones, and the like. A great deal of such images are created by amateur photographers using imaging equipment with limited dynamic range (difference between highest and lowest values of a variable quantity such as light intensity or frequency), under unsuitable and/or non-optimal lighting conditions.
One of the image quality shortcomings in many images, such as photographs, is that when a poorly lit scene is captured, some of the original scene detail is lost in the regions of the image that are dark (shadows) and/or bright (highlights) compared with other regions of the image. This is particularly notable when a scene contains mixed lighting where there are combinations of shadows, highlights, and medium regions that are captured in a single image. This shortcoming may be caused by the limited dynamic range and/or imperfect exposure settings of the image capture device, among other causes.
A variety of techniques have been used to address such shortcomings. One technique to enhance an image is to adjust the lighting of the original scene so that there is limited dynamic range in the scene that is to be captured, for example, by eliminating overly dark and/or overly bright areas in the scene. Other than in a studio where lighting may be adjusted professionally, adjusting the lighting of a scene may be difficult at best and impossible at worst because of lack of control of the scene, equipment, knowledge or any combination thereof.
Another technique to improve an image is to capture the scene with a device capable of representing the full dynamic range of the scene. This technique also presents limited opportunities for improvement due to unavailability of suitable equipment, time constraints, lack of control over the scene or equipment, and the like. Additionally, the quality of a captured image is generally not known until after it has been captured.
Still another technique to enhance a captured image is to post-process the captured image to enhance the detail of the overly dark and/or bright regions of the image. Processing an image after image acquisition is usually possible and desirable in most circumstances because the constraints for post-processing are fewer. However, most conventional image processing methods need a great deal of computational resources, such as processing power and memory. Furthermore, such conventional image processing techniques typically produce undesirable visual artifacts and/or may only address the enhancement of the shadow/dark regions or the highlight/bright regions, but not both in the same image.